In standard medical care, physicians select treatments for patients based on clinical judgment, considering which treatment is best for the individual patient, given the patient's history and circumstances. In contrast, investigators conducting randomized clinical trials select treatments for participants based on a random selection process. Because this process represents a significant departure from the norms of standard medical care, it is widely assumed that potential research participants must understand randomization to give valid informed consent. This assumption, together with data that many research participants do not understand randomization, implies that randomized clinical trials often fail to obtain adequately informed consent. Before accepting this conclusion, and before initiating extensive efforts to improve research participants' understanding of randomization, we should assess the plausible, but rarely analyzed assumption that participants need to understand randomization to give valid informed consent for randomized clinical trials.